Despite tremendous strides in transplantation over the past 60 years, with great advances in short-term graft survival, long-term survival is comparatively poor. This is likely due to the fact that immunosuppressive strategies, though clearly improved over the years, are not tolerogenic. True transplantation tolerance is likely to arise not from improved immunosuppression, but from developing novel strategies based on improved understanding of the physiologic mechanisms which maintain self-tolerance, and the barriers that mediate resistance to tolerance. Moreover, approaches for transplantation tolerance are likely to be applicable for the treatment of many autoimmune disorders as well. Our approach is based on the premise that the mechanisms which mediate peripheral self-tolerance may prove most easily exploited to generate tolerance alloantigens in adults. The overall theme is that the availability of novel t cell activation/differentiation signals, the cytokine milieu, and the presence or absence of inflammation collectively determine whether T cells will be directed towards regulation or effector/memory functions, and thus these parameters determine the ultimate outcome of the alloimmune response;rejection versus tolerance. The aims of project #1 are designed to test the central hypothesis that the TIM-1 :TIM-4 pathway, by affecting Th1/Th2 cell differentiation, generation of effector/memory T cells, and possibly regulatory T cell generation and function, plays an important role in alloimmune and autoimmune responses and tolerance in the setting of islet transplantation. The aims of project #2 are focused on the concept that two events occurring very early during the process of engraftment, inflammation and antigen encounter, determine graft outcome and tolerance susceptibility by modulating the balance between regulatory cells and effector/memory cells. Lastly, project #3 will study how the cytokine milieu determines the balance between the development of pathogenic T cells and tolerogenic regulatory cells. Together, these investigations should provide new insights into how alloimmune T cell responses are initiated and controlled. The results should yield useful new information that can be harnessed to develop novel tolerogenic strategies to test in primates and ultimately humans.